The present invention relates to ECG devices, and in particular to an ECG circuit which utilizes optoisolators in combination with an isolation operational amplifier to provide a safe, simple, reliable, and efficient means of extracting ECG signal information.
Electrocardiography (ECG) is the recording and examination of electrical signals which accompany the contraction of the heart or other muscle. Electrocardiograph electrodes are attached at desired locations on the body and the electrical activity generated by contraction of the heart muscle, as measured on the electrode leads, is connected to and recorded on an electrocardiograph in a conventional manner. However, because the signals thus generated are typically at low voltage levels, amplifier circuits coupled to the electrodes are needed in order to prevent masking of the signals by other extraneous electrical signals (noise). One technique known in the art for providing this amplifying function is to use a differential amplifier. A differential amplifier measures and amplifies only the difference between two input signals. Thus, if a common background signal is present in both input signals, the back-ground signal will not be amplified because it is not part of the difference between the two input signals, but rather represents a common signal to both input signals. The rejection (non-amplification) of such a common signal is referred to in the art as common mode rejection. For ECG applications, the two inputs of a differential amplifier are respectively connected to the selected two points between which the ECG potential difference is to be measured. The differential amplifier advantegously amplifies this ECG potential difference to a much greater degree than other common mode signals that may be present, such as the 60 Hz power line signals that are commonly induced in the body. Another technique used for reducing the effect of external noise signals on the measured heart signal is to use shielded leads. That is, the signal leads connected to the electrodes are surrounded by a metallic screen or foil that is connected to ground or other fixed potential. This latter shielding technique is also utilized in the circuit of the present invention, but with the shield actively driven as will be shown.
Although these efforts to discriminate between the desired electrical voltage signal generated by the heart muscle and the electrical effects of the environment have generally been satisfactory, it is sometimes desirable to provide further discrimination between the sensed electrical signals. For example, modern ECG devices typically utilize multiple transformers on each electrode lead in conjunction with an applied carrier frequency in order to achieve a high common mode rejection and a high isolation factor. Over-voltage discharge elements are also utilized to protect the circuitry against high voltages, which high voltages may be present in the event a defibrillator device is used on the patient. A high isolation factor, which prevents currents from flowing from one part of the circuit to another where current is not intended to flow, is necessary to both protect the patient if the patient inadvertently is connected to ground or a high potential, such as a power line (thus forming a closed circuit loop), and in order to achieve an improved high common mode rejection of extraneous signals, such as the 60 Hz power line signal, thereby allowing the ECG signal to be more cleanly amplified and recorded or displayed.
Although the use of transformers on each patient lead provides a circuit having high common mode rejection and a high isolation factor, the transformers are expensive, bulky, and tend to be unreliable. Accordingly, what is needed is to provide an improved circuit for use with ECG devices that provides high common mode rejection, a high isolation factor (has low leakage current), and that is reliable, compact and relatively inexpensive. It is further desired that this circuit have the capability of sustaining without damage large voltage surges such as might occur if the patient requires defibrillation during recording of the ECG signal. Moreover where multiple electrode leads are placed on the patient, it is desirable to be able to selectively connect the ECG device between selected electrodes or combinations of electrodes.